I live in sunny So Cal and never pay much attention to the temp gage unless I am towing and the 6.0 starts running hot.
Over the holidays I took a trip with the family to Washington and Central Oregon, and had a very unusual problem with the 6.0.
It was very cold and for some reason the 6.0 would not heat up. I could not even get the needle on the gage to the first line. The heater would only blow cold air.
I assumed that the Thermostat must be stuck wide open and would not allow the antifreeze to stay in the block long enough to get warm. I drove around 700 miles in these two states and never had a heater or an engine running at proper temp.
When I got back to So Cal, I pulled out the old Thermostat and replaced it with a new one. The old unit looked to be perfectly fine and was closed when I pulled it out.
Is there anyone out there that may have had a similar issue and knows what else may be keeping the 6.0 from reaching proper temp? I am at a loss as to where to look next.

Unfortunately you can not rely on the dash gauge. You need to get something to monitor your temperatures so you can see what the various temperatures actually are (Oil and coolant being very important)

I forgot to mention that I did hook up the temp to a computer reader from my mechanic. We checked it at idle after the 6.0 had been driven for a while. It was probably high 70's outside and the temp only reached to 114 degrees with the AC running. I believe the stock thermostat is set to open at 100C I believe that is around 140F. There is no reason why the engine should have a hard time achieving that temp with a closed thermostat.
If the issue was the gauge, it still does not answer the question as to why the heater only would blow cold air when we were in the cold temps all day long. We drove a minimum of 40 miles a day and sometimes over 200 while it was cold. The heater never blew anything but very cold air. When we got back to CA and warmer temps the heater worked fine.
Any thoughts?

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